Selectivity enhancement of p-type semiconducting hydrocarbon sensors - the use of sol-precipitated nano-powders

Publisher’s version / Version de l'éditeur:

Sensors and Actuators B: Chemical, 130, 1, pp. 470-476, 2008-03-14

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.1016/j.snb.2007.09.024

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Selectivity enhancement of p-type semiconducting hydrocarbon

sensors - the use of sol-precipitated nano-powders

Sahner, K.; Schönauer, D.; Matam, M.; Post, M.; Moos, R.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=be4afc42-8820-45ce-bda5-f953ce193796 https://publications-cnrc.canada.ca/fra/voir/objet/?id=be4afc42-8820-45ce-bda5-f953ce193796

Sensors and Actuators B 130 (2008) 470–476

Selectivity enhancement of p-type semiconducting hydrocarbon

sensors—The use of sol-precipitated nano-powders

K. Sahner

a,∗

, D. Sch¨onauer

a

, M. Matam

b

, M. Post

b

, R. Moos

a

a_{Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany}

b_{Institute for Chemical Process and Environmental Technology, National Research Council of Canada,} Ottawa, Ontario K1A 0R6, Canada

Available online 16 September 2007

Abstract

A method for preparing sol-precipitated Sr(Ti0.8Fe0.2)O3−δ powders yielding particle sizes of about 70 nm is presented. Screen-printed films based on such nano-scaled powders are compared and contrasted to conventional micro-scaled thick films.

By employing sol-precipitated powders, the hydrocarbon sensitivity of the sensor devices is improved. For unsaturated hydrocarbons, resistance changes similar to those of thin films prepared by pulsed laser deposition are observed, thus combining the fast sensor response of Sr(Ti0.8Fe0.2)O3−δ thick films with the high response (R/R0) values observed for thin films. The pronounced cross-interference of NO and CO can be eliminated by means of an additional zeolite layer deposited on top of the nano-scaled sensor film.

© 2007 Elsevier B.V. All rights reserved.

Keywords: Sol precipitation; Zeolite; Pulsed laser deposition; Hydrocarbon sensor; Sr(Ti0.8Fe0.2)O3−δ

1. Introduction

More stringent environmental regulations all over the world have increased the demand for inexpensive, accurate and reli-able gas sensor devices. There are many examples of toxic or explosive gases that should be monitored, and these include CO, nitrous oxides, ammonia, hydrocarbons, hydrogen, etc.[1–3].

For cars equipped with a three-way catalyst, the need for a reliable “On-Board Diagnosis” (OBD) of the exhaust gas aftertreatment system might enforce an additional sensor down-stream of the catalyst. To date, the catalyst functionality is monitored indirectly by correlating the oxygen storage capac-ity with the conversion efficiency. A decrease in the conversion rate and oxygen storage capacity of the catalyst results in an increase in the amplitudes of the ␭-oscillations downstream from the three-way catalyst. The ratio of the amplitudes of ␭-probes located upstream and downstream from the catalyst is thus a measure that can be used to evaluate catalyst efficiency[3]. In the future, however, a more direct and accurate method will be

∗_{Corresponding author at: Functional Materials, University of Bayreuth,}

Uni-versit¨atsstr. 30, D-95447 Bayreuth, Germany. Tel.: +49 921 55 7400; fax: +49 921 55 7405.

E-mail address:Functional.Materials@uni-bayreuth.de(K. Sahner).

needed to determine whether or not the exhaust aftertreatment system of cars with lowest emissions meets the emission require-ments. Therefore, a direct measurement method employing a sensor that detects the amount of unburnt hydrocarbons is desir-able[4]. A hydrocarbon sensor can also be used for On-Board Diagnosis of close-coupled oxidation catalysts in diesel engines

[5], or for safety applications, e.g. in Japan, where most of the households are equipped with propane-based heating systems.

Although n-type conducting metal oxides like SnO2or ZnO

dominate the research effort in this sensor area, some p-type con-ducting ceramics have recently received more attention as well. These exceptions include the commercially available ammonia gas sensors, Cr2−xTixO3[6,7]and NOx sensors based on

per-ovskite rare-earth metal oxides of the LnFeO3family, where Ln

is La, Nd, Sm, Gd and Dy[8].

The perovskite family SrTi1−xFexO3−δ with x = 0.1–0.5 are

promising materials for hydrocarbon sensing in the tempera-ture range from 350 to 450◦_{C. With screen-printed films of}

about 20 ␮m thickness, a fast, stable and reversible response to hydrocarbons is observed. In particular, the SrTi1−xFexO3−δ

members with a moderate iron content (x = 0.1–0.2) show best performance[9,10].

The present work addresses selectivity enhancement of SrTi0.8Fe0.2O3−δ based sensors. Whereas an earlier paper

focused on an extrinsic method using an additional zeolite

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.024

K. Sahner et al. / Sensors and Actuators B 130 (2008) 470–476 471

filter layer[11], the present contribution takes a closer look at the precursor material itself. As the sensor effect towards reducing gases includes a mechanism which is attributed to surface pro-cesses, i.e., a redox reaction taking place at the semiconductor surface, the sensitivity of a given material should be improved by enhancing the surface area-to-volume ratio. This can be achieved either by fabricating thin film devices or by using nano-scaled materials[12 (and references therein)–14], such as fine-grained precursor powders.

In the following, a wet-chemical synthesis method for prepar-ing nano-sized SrTi1−xFexO3−δ powders is described. The

sensor performance of thick films prepared from these precur-sors is evaluated. The functionality of the nano-sized particles is compared to micro-scaled powders obtained by a conventional solid-state reaction and to thin film devices.

2. Experimental

Conventional powders of the p-type semiconductors Sr(Ti0.8Fe0.2)O3−δwere prepared from the precursor materials

SrCO3(Technipur, Merck, Germany), TiO2(Merck, Germany)

and Fe2O3(AlfaAesar, Johnson Matthey GmbH, Germany)

fol-lowing the mixed oxide route. After calcination at 1200◦_C,

the powders were ball-milled. By laser diffraction (Mastersizer 2000, Malvern Instruments), a particle size of 1.7 ␮m d50was

determined. These rather coarse powders, which serve as base materials for screen-printing pastes and targets for pulsed laser deposition, are referred to as MO powders in the following.

The synthesis of sol-precipitated nano-powders followed a titanate preparation route described by Zheng et al.[15]with a slight modification. As precursors, Sr(NO3)2, Fe(NO3)3·9H2O

and titanium isopropoxide (all Fluka) were used in stoichio-metric proportions. The precipitation reaction was conducted in continuously stirred aqueous solution. The pH value was adjusted using glacial acetic acid. In contrast to the original method, no 4 M NaOH was added at this point of the synthesis. The solution was heated to 80◦_{C and kept at this temperature for}

3.5 h. Then powder precipitation was initiated by addition of 4 M NaOH. The reaction product, which is denoted as SP powder in the following, is subsequently calcined at 850◦_{C for 3 h.}

In both cases, phase purity of the samples was assured by XRD measurements (Phillips X’Pert-MPD-PW3040/00) with monochromated Cu K␣ radiation (10◦

≤2Θ ≤ 90◦). Invariably, all peaks of the obtained XRD spectra could be indexed assuming a pseudo-cubic perovskite lattice.

The interdiffusion of elemental components of Sr(Ti1−xFex)

O3−δthick films and alumina substrates is a known problem that

can be eliminated by applying a SrAl2O4underlayer between

the alumina substrate and the electrodes as described elsewhere

[16]. According to former results [10], the use of platinum for electrical contacting is very advantageous. Hence, either a set of four platinum electrodes or two platinum interdigitated electrodes (IDE) were applied onto the substrate prior to Sr(Ti1−xFex)O3−δ film deposition. After sintering at 1100◦C

(MO powders) or 850◦_{C (SP powders) in air for 1 h, the film}

thickness was determined by a profilometer to be between 10 and 20 ␮m. The sensor configuration is shown inFig. 1.

Fig. 1. Diagrammatical presentation of the thick film sensor devices includ-ing the dimensions of the active thick film sensor layer. As an alternative, interdigitated Pt electrodes (100 ␮m line and spacing) were used.

For the pulsed laser deposition (PLD) of thin films, a pressed pellet was prepared from the MO powders and densely sintered at 1520◦_{C. This as-prepared target was used to deposit thin}

films on substrates of polycrystalline alumina equipped with a strontium aluminate film and platinum IDEs. An excimer laser (KrF Lambda Physik, LPX305i) with wavelength λ = 248 nm, at a pulse frequency of 8 Hz, and energy of 600 mJ was used to deposit the films. The energy fluence at the target was estimated to be ∼1.6 J/cm2. Depositions were carried out under an oxygen atmosphere at a partial pressure of 13 Pa, with a heated substrate held isothermally at 700◦_{C. After annealing at the same}

tem-perature for 30 min under an oxygen partial pressure of 53 kPa the films were cooled to room temperature at 10◦_{C/min. The}

thickness of the films was estimated to be 200 nm from previous calibrations.

For determining sensor functionality, the specimens were either heated to the corresponding operating temperature in a tube furnace, or they were mounted directly in a sample cham-ber and heated via a platinum thick-film heater structure on the sensor backside. In the entire study, the temperature range from 350 to 500◦_{C was covered. In the present paper, sensor tests}

focused on measurements at 400◦_C.

Using mass flow controllers, different analyte gases were added to a reference nitrogen flow containing 20 vol% of oxy-gen. Total gas flow was adjusted either to 600 ml/min for the sensors in the tube furnace or to 6 l/min in the case of the actively heated sensors. Before each measurement, samples were heated at 400◦_{C for several hours to allow for thermal}

equilibra-tion. Before adding analytes gases, the furnace or the sample chamber was purged with the reference composition of 20% oxygen in nitrogen for at least 20 min until a stable baseline was observed.

Hydrocarbon (HC) sensitivity was measured for propane (300–2500 ppm) and propene (80–1000 ppm). In order to compare the effect of hydrocarbons with different reactivity or a different chain length, sensor response towards methane, propane, propene and butane was also investigated. To assure comparability, the concentration of these different hydrocarbon species CnHm was chosen so that total concentration of

carbon atoms cTCA=n•cCnHmwas kept constant at 1500 ppm. Hence, cmethane= 1500 ppm, cpropane= cpropene= 500 ppm and

(HC mix, 375 ppm) containing ethane, ethene, acetylene and propene in equal proportions was tested.

In order to study response to interference gases, sim-ilar measurements were conducted with CO, NO (both 500–750–1000 ppm) and H2(5000–2500–1000 ppm). In

addi-tion to the actual values of the mass flow controllers, the gas composition was verified analytically using an FTIR spectrome-ter (except for H2). The dc-sensor resistance of all sensor devices

was monitored with a Keithley 2700 digital multimeter. 3. Results and discussion

Fig. 2a and b presents SEM micrographs (Leo 1450 VP) of thick films prepared from different base powders, both taken at the same magnification. The MO powders (Fig. 2a) obtained via a solid-state reaction are very porous and coarse-grained with grain sizes of up to 1.2 ␮m after film sintering. By the wet-chemical route, however, very fine powders are obtained. The grain diameters are about 70 nm (Fig. 2b). At a lower magni-fication (Fig. 2c), however, one notes a coagulation of the fine particles into larger agglomerates. These coarse secondary par-ticles are observed only after calcinations. Such a formation of agglomerates is often observed for nano-powders and an impor-tant problem in nano-sintering[17]. In the case of porous thick films, however, there is no need to avoid the coagulation process since the surface of the single particles is still accessible to gas exposure.

Additional multi-point BET measurements (Micromeritics ASAP 2010, adsorptive: N2) conducted on the powders yielded

specific surface areas of 3 m2/g for conventional MO powders

Fig. 3. Comparison of the normalized sensor signals of a conventional (dashed line) and of a nano-scaled SP-based (solid line) Sr(Ti0.8Fe0.2)O3−δ sensor at

400◦_{C in dry air (upper part). Analyte gas concentrations as indicated (lower}

part).

and up to 20 m2/g for SP powders, thus indicating a smaller grain size of the SP particles. These findings are in agreement with the SEM results.

Typical conductometric response curves of a conventional MO-based sensor device (dashed line) and a thick film sensor prepared from sol-precipitated powder (solid line) are shown in

Fig. 3. For comparing the sensor characteristics of the sensor films, the resulting resistance change response R/R0(as defined

in ref.[18]) are plotted.

Fig. 2. SEM micrographs of a conventional sensor film (a) and of a fine grained sol-based film (b). In addition, an SEM micrograph of the SP film at lower magnification is given in (c).

K. Sahner et al. / Sensors and Actuators B 130 (2008) 470–476 473

Fig. 4. Comparison of the normalized sensor signals of micro-scaled (left) and nano-scaled SP-based (right) Sr(Ti0.8Fe0.2)O3−δsensors at different temperatures in

dry air. The lines have been inserted as a guide to the eye. Analyte gas concentrations: 500 ppm propane and 500 ppm propene, respectively.

Both sensors show a fast, reversible and reproducible response towards propane and propene, and each has a very stable baseline. The base resistance in air, R0, of SP powders

(in the order of 300–400 k at 400◦_{C) was found to be higher}

than that of MO powders (in the order of 1–3 k at 400◦_C).

This is attributed to the fine-grained microstructure of the SP sensor films, where the firing temperatures are very low, and consequently almost no sintering occurs. Hence, current path-ways are passing through many narrow interparticle contacts or “necks”, which contribute to total resistivity, and high resistance values are observed.

Whereas the conventional MO powder reaches a maximum change R/R0of only 3.5 when exposed to 1000 ppm propene,

the nano-scaled SP-based sensor shows a more pronounced response for both hydrocarbons. In this case a maximum R/R0

value of 11.2 is observed during propene exposure. Minor cross-interference of NO, CO and H2is observed, whereas the

con-ventional device only presents a cross-sensitivity towards NO. Thus, the hydrocarbon sensitivity of an Sr(Ti1−xFex)O3−δ

thick film device is improved by applying nano-scaled SP-based materials. As shown inFig. 4for 500 ppm propane and propene, this trend was valid over the temperature range from 350 to 500◦_{C. Keeping in mind the tradeoff between the slower}

response at low temperatures and the lower signal at high temper-atures (cf.Fig. 4), the following sections focus on measurements at 400◦_{C, which had been identified previously as the optimum}

operating temperature[9].

A mechanistic model describing this phenomenon, which is attributed essentially to the enhanced surface-to-volume ratio, has been proposed elsewhere[19,20]. A method to reduce the cross-sensitivities by means of a zeolite layer as reported in ref.

[11]is discussed below.

Sensitivity m, which describes the ratio of the change in sen-sor output to the change in the value of the measurand, was also determined using the following power law:

R ∝ cm_HC (1)

where cHCis the hydrocarbon concentration and R is the sensor

resistance.

To determine the value of m, sensor response measurements were conducted in the range from 100 to 2000 ppm (propane) and 80 to 1000 ppm (propene). Fig. 5shows a double-logarithmic representation of the results of the sensitivity tests for propane and propene, where log R = f(log cHC). This analysis yields a

linear relationship with the slope m, which is commonly used as a measure of sensitivity. For both hydrocarbon species, the use of a fine-grained precursor powder leads to a notable increase in sensor sensitivity. In the case of propane, m increases from 0.22 to 0.4, whereas propene sensitivity changes from m= 0.25 to 0.38. This is in agreement with findings reported for other semiconducting sensor materials, where a decrease in size leads to enhanced sensitivity[14,21]. As the sensing behav-ior of perovskite semiconductor sensors is attributed to a redox reaction taking place at the sensor surface, the enhanced sur-face/volume ratio increases the role of surface states in the sensor response.

In Fig. 6, typical relative resistance R/R0 values of

Sr(Ti0.8Fe0.2)O3−δsensors are presented for a variety of

hydro-carbons. Thick films prepared from MO and SP powders are compared to PLD thin films.

According to these results, the performance of sol-precipitated powders ranges between conventional MO thick films and thin film sensors for unsaturated hydrocarbons and long HC chains. For short saturated HC species such as methane or propane, where the sensitivity of thin films is reported to be low[10], the SP device performs best. Integration of PLD thin films and SP based thick films on a single sensor chip could thus lead to a very sensitive hydrocarbon sensor, which in addition has the potential to discriminate between saturated and unsaturated hydrocarbons.

InTable 1, t90values, i.e., the time at which the sensor output

reaches 90% of its final value, are summarized for the different Sr(Ti0.8Fe0.2)O3−δsensors at 400◦C in response to a step change

Fig. 5. Propane (left) and propene (right) sensitivity of Sr(Ti0.8Fe0.2)O3−δsensor films based on MO and SP precursor powders at 400◦C.

Fig. 6. Relative resistance R/R0 of Sr(Ti0.8Fe0.2)O3−δsensor films at 400◦C.

Gases and concentrations as indicated in dry air. R0values: 1.46 k (MO

pow-der), 349 k (SP powder) and 1320 k (PLD, thin film).

The response times of SP based thick films are comparable to those for the MO powders and are faster than the responses found for PLD thin films. Thus, the nano-scaled thick film devices combine the fast sensor response of Sr(Ti0.8Fe0.2)O3−δ thick

films with the high R/R0values observed for thin films.

Table 1

Response times t90of different Sr(Ti0.8Fe0.2)O3−δsensor films at 400◦C towards

500 ppm propane and propene

t90(s)

MO thick film SP thick film PLD thin film Propane, 500 ppm 85 150 >975a

Propene, 500 ppm 155 130 255

a_{After 20 min, the thin film had not reached a stable plateau value. The}

pre-sented t90 value has been calculated using the final reading before propane

injection was stopped.

By covering a nano-scaled Sr(Ti0.8Fe0.2)O3−δ film with

a 50 ␮m thick Pt-doped ZSM5 (1.6 wt% of Pt, cation ratio Na+/H+= 85:15) layer, one obtains a very sensitive and selec-tive hydrocarbon sensor as shown in Fig. 7. Compared to a conventional MO thick film sensor (dashed line), the sensor performance is considerably improved with respect to both sensitivity and selectivity. By applying the zeolite layer, the cross-interference of NO and CO has been effectively elimi-nated. This is because the ZSM5 zeolite is an excellent oxidation catalyst, and while passing through the zeolite layer, CO is oxi-dized to CO2prior to reaching the sensor layer[11].In the case

Fig. 7. Relative resistance R/R0of a zeolite-covered Sr(Ti0.8Fe0.2)O3−δSP film

and a MO film at 400◦_{C. Gases as indicated in dry air. R}

0values: 2.80 k (MO

K. Sahner et al. / Sensors and Actuators B 130 (2008) 470–476 475

of NO, one part of the interfering gas is converted to NO2

accord-ing to the correspondaccord-ing NO–NO2equilibrium. This conversion

was verified with additional catalytic tests on the zeolite pow-der. In both cases, response of the Sr(Ti0.8Fe0.2)O3−δ film is

suppressed because the analyte species does not reach the sen-sor layer. Noteworthy is the increase of propane sensitivity that is observed when the zeolite layer is present. This observation is currently being investigated and possibly due perhaps to partial oxidation of propane to some other active compound.

In an additional measurement series, the impact of humidity on propane sensitivity of sensors with and without the zeolite layer was investigated. For this purpose, 2.5 vol% of water vapor was added to the reference gas flow. The presence of humidity was found to suppress the propane sensitivity of the bare sensor films to some extend. At 500 ppm, for example, the sensor signal decreased from 1.6 to 1.3, no cross-interference was observed for the zeolite covered layers (for details see ref.[19], addi-tional results will be discussed in a forthcoming paper). This was attributed to the fact that water vapor tends to adsorb in the zeolite channels, so that it does not reach the sensor surface. The zeolite layer thus enables the effective reduction of various cross-interfering species.

4. Conclusion

Sol-derived nano-scaled Sr(Ti0.8Fe0.2)O3−δpowders can be

used as precursors for screen-printed p-type conductometric hydrocarbon sensors. Such devices show a fast and sensitive response, and the signal is found to be stable and reproducible. In addition, they perform better than conventional micro-scaled thick films over the entire temperature range from 350 to 500◦_C.

The size of the precursor powders for Sr(Ti1−xFex)O3−δ

sen-sors is a crucial parameter in order to improve sensor functional qualities. Further investigation will therefore be directed to other synthesis methods for nano-materials, such as electro-spraying or electrospinning, in order to confirm these promising results.

As another approach for selectivity enhancement, the use of PLD thin films, which present a good selectivity towards unsaturated hydrocarbons such as propene, was tested suc-cessfully. Finally, the use of a catalytically active zeolite cover layer provided a further selectivity enhancement by effectively reducing the cross-interference of NO and water vapor.

Acknowledgements

The authors thank Ms. Monika Wickles for sample prepa-ration, Mr. Hans-J¨urgen Deerberg for SEM micrographs, Mrs. Ingrid Otto and Mr. Hannes Wolf for BET measurements and Dr. Andreas Dubbe and Mr. Gunter Hagen (all University of Bayreuth) for zeolite preparation.

Financial support of this project was provided through a joint international program of the National Research Council of Canada, the Helmholtz Gemeinschaft and German Federal Ministry of Education and Research (BMBF) and is gratefully acknowledged (Projects NRCC-21-CRP-02 and 01SF0201 9.2).

References

[1] N. Docquier, S. Candel, Combustion control and sensors, a review, Prog. Energy Combust. Sci. 28 (2002) 107–150.

[2] B. Timmer, W. Olthuis, A. van den Berg, Ammonia sensors and their applications—a review, Sens. Actuators B 107 (2005) 666–677. [3] J. Riegel, H. Neumann, H. Wiedenmann, Exhaust gas sensors for

automo-tive emission control, Solid State Ionics 152–153 (2002) 783–800. [4] R. Moos, A brief overview on automotive exhaust gas sensors based on

electroceramics, Int. J. Appl. Ceram. Technol. 2 (2005) 401–413. [5] T. Moser, F. Stanglmeier, B. Schumann, S. Thiemann-Handler, Sensor

in planarer Dickschichttechnik zur Messung von Kohlenwasserstoffen im Abgas von Kraftfahrzeugen (Planar thick-film sensor for measuring hydro-carbons in automotive exhausts), in: VDI-Berichte 1530, VDI-Verlag, Ludwigsburg, Germany, 2000, pp. 159–167.

[6] D. Niemeyer, D.E. Williams, P. Smith, K.F. Pratt, B. Slater, C.R.A. Catlow, A.M. Stoneham, Experimental and computational study of the gas-sensor behaviour and surface chemistry of the solid-solution Cr2−xTixO3(x < 0.5),

J. Mater. Chem. 12 (2002) 666–675.

[7] Y. Li, W. Wlodarski, K. Galatsis, S.H. Moslih, J. Cole, S. Russo, N. Rock-elmann, Gas sensing properties of p-type semiconducting Cr-doped TiO2

thin films, Sens. Actuators B 83 (2002) 160–163.

[8] G. Martinelli, M.C. Carotta, M. Ferroni, Y. Sadaoka, E. Traversa, Screen-printed perovskite-type thick film gas sensors for environmental monitoring, Sens. Actuators B 55 (1999) 99–110.

[9] K. Sahner, R. Moos, M. Matam, J. Tunney, M. Post, Hydrocarbon sens-ing with thick and thin film p-type conductsens-ing perovskite materials, Sens. Actuators B 108 (2005) 102–112.

[10] K. Sahner, R. Moos, M. Matam, M. Post, Effect of electrodes and zeolite cover layer on hydrocarbon sensing with p-type perovskite SrTi0.8Fe0.2O3−δthick and thin films, J. Mater. Sci. 41 (2006) 5828–5835.

[11] K. Sahner, R. Moos, M. Matam, M. Post, Use of a zeolite cover layer for improving sensor characteristics of p-type conducting perovskites, in: Sensor 2005 Proceedings, vol. I, Nuermberg, Germany, May 2005, pp. 201–206.

[12] P.I. Gouma, Nanostructured polymorphic oxides for advanced chemosen-sors, Rev. Adv. Mater. Sci. 5 (2003) 147–154.

[13] V. Guidi, M.C. Carotta, M. Ferroni, G. Martinelli, L. Paglialonga, E. Comini, G. Sberveglieri, Preparation of nanosized titania thick and thin films as gas-sensors, Sens. Actuators B 57 (1999) 197–200.

[14] E. Comini, G. Faglia, G. Sberveglieri, D. Calestani, L. Zanotti, M. Zha, Tin oxide nanobelts electrical and sensing properties, Sens. Actuators B 111–112 (2005) 2–6.

[15] H. Zheng, X.Q. Liu, G.G. Meng, O.T. Sorensen, Fine SrTiO3 and

Sr(Mg0.4Ti0.6)O3−δ perovskite ceramic powders prepared by a

sol-precipitation process, J. Mater. Sci.: Mater. Electron. 12 (2001) 629–635. [16] R. Moos, F. Rettig, A. H¨urland, C. Plog, Temperature-independent resistive

oxygen exhaust gas sensors for lean-burn engines in thick-film technology, Sens. Actuators B 93 (2003) 43–50.

[17] J.R. Groza, Nanosintering, Nanostruct. Mater. 12 (1999) 987–992. [18] A. D’Amico, C. Di Natale, A contribution on some basic definitions of

sensor properties, IEEE Sens. J. 1 (2001).

[19] K. Sahner, Modeling of p-Type Semiconducting Perovskites for Gas Sensor Applications, Shaker Verlag Aachen, Germany, 2006, ISBN 3-8322-5538-9.

[20] K. Sahner, R. Moos, Modeling of hydrocarbon sensors based on p-type semiconducting perovskites, Phys. Chem. Chem. Phys. 9 (2007) 635–642. [21] G. Bl¨aser, Th. R¨uhl, C. Diehl, M. Ulrich, D. Kohl, Nanostructured semicon-ductor gas sensors to overcome sensitivity limitations due to percolation effects, Phys. A: Stat. Theor. Phys. 266 (1999) 218–223.

Biographies

Kathy Sahnerreceived her German engineering diploma in materials science from the Saarland University, Germany, in 2002. At the same time, she also received the corresponding French degree from the European School of Materi-als Science and Engineering (EEIGM), France. During her PhD, she investigated

of gas sensitive perovskites including mechanistic modeling. She then worked as a postdoctoral researcher at the University of Bayreuth, Germany, and at the Massachusetts Institute of Technology, USA, on various projects within the field of electroceramics. Her current research interests focus on materials for gas sensor applications, transport phenomena in sensor films, and ion conductors.

Daniela Sch¨onauerreceived her engineering diploma in materials science in 2005 from University of Bayreuth, Germany. Since 2006, she is a PhD student at the chair of Functional Materials at the University of Bayreuth. Her research interests are gas sensor applications and exhaust gas aftertreatment systems.

Mahesh Matam after obtaining his PhD from India in ferroelectrics/ piezoelectric ceramics, worked at Simon Fraser University (SFU), Vancou-ver as a post doctoral fellow. At SFU he grew single crystals and thin films of piezoelectric materials before moving to the National Research Council of Canada (NRCC) in Ottawa as a Visiting Fellow. His focus at NRCC was on growing and characterizing thin perovksite oxide films for hydrocarbon sensing using pulsed laser ablation. In 2006, he moved into the piezo-ceramics indus-try where he currently works on R&D projects related to ultrasonic sensors. He works for Piezotechnologies in Indianapolis, Indiana and can be reached at

mmatam@piezotechnologies.com.

Michael Postreceived his PhD in chemistry from the University of Surrey, UK, in 1971 and is a Principal Research Officer and leader of the sensor and devices project at the ICPET institute of the National Research Council of Canada, where he has been an active researcher in materials science since 1975. Projects have included X-ray diffraction and structure determination, intermetal-lic compounds for hydrogen storage and phase studies of high temperature superconducting ceramics. Recent research interests are directed toward the investigation of structural and functional relationships of thin and thick film non-stoichiometric compounds and nanomaterial composites for application as gas sensors.

Ralf Moosreceived his diploma in electrical engineering in 1989 from Univer-sity of Karlsruhe, Germany. As a PhD student, he conducted research on defect chemistry of titanates. In 1995, he joined DaimlerChrysler and worked in the serial development of exhaust gas aftertreatment systems. In 1997, he moved to DaimlerChrysler Research in Friedrichshafen, where he headed several projects in the field of exhaust gas sensing. Since 2001, he is head of the Chair of Func-tional Materials of the University of Bayreuth. His main research interest are materials and systems for exhaust gas aftertreatment, gas sensing, and gas sensor technology.